NEW YORK (CNN) -- Power slowly flickered on across the northeastern United States and parts of southern Canada after the mysterious massive outage cut electricity Thursday afternoon from New York north to Toronto and west to Detroit -- an area home to some 50 million people.

Times Square, for many the glittering symbol of New York under normal circumstances, sprang back to life about 7:45 a.m. EDT.

The North American Electric Reliability Council (NERC), which monitors the power system, said that an approximate 41,100 megawatts of generated electricity, out of 61,800 megawatts lost, had been restored by 5 a.m. EDT.

With most of his state back on line -- Long Island and parts of five New York City boroughs the exceptions -- New York Gov. George Pataki said that a power failure of this magnitude "shouldn't have happened."

"We have to have answers to this," he said. "We are an energy-dependent society."

Electric regulatory agencies have some "tough questions" to answer about where, how and why a cascading blackout shut down parts of the U.S. Midwest and Northeast and Canada, Pataki said Friday. (Newspapers struggle to print, air and rail travel recovering)

After the last major outage blacked out the city in 1977, a system of safeguards was put in place to prevent such events.

"Where did this happen? How did it happen?" Pataki said. "Given the safeguards that were put in place ... why did the system fail?"

Pataki said earlier reports that a major power plant at Niagara had been the source of the outage were "completely inaccurate."

NERC said in a statement that the problem "appears to have been largely caused by the loss of several major power lines in the upper Midwestern United States."

The Associated Press reported Friday that indications pointed somewhere along Lake Erie in Ohio, citing the industry-sponsored group that monitors the transmission system.

"That's where the information is starting to point,'' Ellen Vancko, a spokeswoman for NERC, told The Associated Press. She said it would take time to pinpoint the cause.

Pataki said for the moment, the state's main focus was getting the power back on. Millions of New Yorkers are still without power, he said.

Parts of four New York boroughs and all of Staten Island had power early Friday, a spokesman for the city's Office of Emergency Management said.

Three deaths have been reported in relation to the outage.

New York Mayor Michael Bloomberg said at least one person died as a result of the blackout, and at least one firefighter was injured. Calls came in for some 3,000 incidents of fire, he said, many from people using candles. Emergency services, he said, responded to 80,000 calls to 911 for help, more than double the average. (Low-tech in unplugged NYC)

In Canada, Ottawa Director of Emergency Services Tony Dimanti said a 15-year-old died from injuries suffered in a fire, and another person died at the scene after being hit by a car during an altercation.

Pataki said power officials were bringing the city online slowly to make certain they don't again overload the circuits

Bloomberg urged New Yorkers to stay home and conserve energy.

New York police reported only four burglaries in the entire city overnight, and said they had made arrests in all four.

No fatalities or major injuries were reported, police said, adding that the city's worst problems were with people stuck in elevators and subways. (Newspapers struggle to print)

In addition to New York, outages were reported in Cleveland, Ohio; Detroit, Michigan; Erie, Pennsylvania; Toronto and Ottawa, Canada; Niagara Falls, New York; Niagara Falls, Ontario; and other cities in Connecticut and New Jersey. (Effects at a glance)

The power was cut in just three minutes, as 21 power plants stopped operating, according to Genscape, a company that monitors the output of power plants. The plants, including 10 nuclear plants, shut down between 4:10 p.m. and 4:13 p.m., Genscape said.

Canadian officials insisted the outage originated in America, but U.S. officials and experts were less certain. (Canada and U.S. blame each other)